feat: track anonymous lesson and quiz engagement in analytics

[OpenSauce]

Summary

• Fire lesson_view and lesson_complete analytics events from the lesson page for both anonymous and authenticated users
• Fire quiz_start and quiz_complete analytics events from the quiz page for both anonymous and authenticated users
• Add anonymous engagement query to the analytics summary endpoint, returning counts under a new anonymous_engagement key
• Display an "Anonymous Engagement" section in the admin dashboard with lesson views, quiz starts, and quiz completions

Acceptance Criteria

• lesson_view event fires on lesson page mount (both anon and auth)
• lesson_complete event fires when markComplete() succeeds
• quiz_start event fires on quiz page mount (both anon and auth)
• quiz_complete event fires after submitQuiz() receives a successful result, with score and total in payload
• Analytics summary endpoint returns anonymous_engagement with lesson_views, lesson_sessions, quiz_starts, quiz_completions, quiz_sessions
• Admin dashboard AnalyticsSummary interface updated to include anonymous_engagement
• Admin dashboard displays "Anonymous Engagement" section with stat cards after Learning Metrics and before Learning Funnel

Test plan

• Frontend tests pass (57 tests, no regressions)
• Backend tests pass (157 tests, no regressions)
• Backend ruff lint clean

Deviations from plan

None

Implements Task 74 from the backlog.

[Copilot]

Pull request overview

This PR adds anonymous user analytics tracking for lessons and quizzes, enabling the platform to measure engagement from users who haven't signed up. The implementation tracks key user interactions (lesson views, lesson completions, quiz starts, and quiz completions) through the existing analytics event system, aggregates this data in the backend analytics summary endpoint, and displays it in a new "Anonymous Engagement" section on the admin dashboard.

Changes:

• Frontend: Added analytics event tracking for lesson views/completions and quiz starts/completions using the useAnalytics composable
• Backend: Extended the analytics summary endpoint with a new SQL query to aggregate anonymous engagement metrics from the event table
• Admin UI: Added a new "Anonymous Engagement" section displaying lesson views, quiz starts, and quiz completions with session counts

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
frontend/pages/lesson/[slug].vue	Added lesson_view event on mount and lesson_complete event after successful completion
frontend/pages/quiz/[slug].vue	Added quiz_start event on mount and quiz_complete event after successful submission with score data
frontend/pages/admin.vue	Extended AnalyticsSummary interface and added UI section to display anonymous engagement metrics
backend/app/api/analytics.py	Added SQL query to count anonymous lesson/quiz events and included results in summary endpoint response

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